NEOPROTEROZOIC TO CAMBRIAN RIFT-TO-DRIFT SEDIMENTATION IN THE BLUE RIDGE PROVINCE OF VIRGINIA

A compelling ancient analog to ‘rift-to-drift’ sedimentation along modern rifted continental margins is found in the uppermost Neoproterozoic to Lower Cambrian Chilhowee Group, exposed in a series of discontinuous belts from Alabama to Vermont. Sedimentary units of the Chilhowee Group mark the transition from the final stages of rifting of the supercontinent Rodinia to the establishment of a passive margin along the eastern margin (present-day coordinates) of Laurentia. The Chilhowee Group is exposed in the Valley and Ridge-Blue Ridge transition zone throughout much of Virginia; in SW VA it comprises the Unicoi (lower), Hampton(middle), and Erwin (upper) Formations. Significant exposures occur within the Blue Ridge Scout Reservation (BRSR) in Pulaski County, VA. This area was last explored in detail by McDowell (1968), who mapped the strata in two thrust sheets, the Hiwassee and Powhatan fault blocks. Although McDowell applied formal names to rocks in the Hiwassee fault block (Erwin and Hampton Fms), he only tentatively correlated strata in the Powhatan block as “lower,” “middle,” and “upper” units of the Chilhowee Group.

In a project coordinated with the BSA Blue Ridge Council, the VA Department of Mines, Minerals, and Energy, and EDMAP, we are reevaluating the geology to provide more detailed coverage in anticipation of future development and needs for resource and hazard management. Initial questions raised by our assessment of McDowell’s 1968 map pertain to formational boundaries and structural/fault placement. Field observations and petrographic analyses allow us to confidently assign formational names to the units in the Powhatan block. Based on the presence of the trace fossil Skolithosin rocks previously labeled “lower Chilhowee” quartzite, we can reassign this formation to the upper Chilhowee unit, the Erwin Fm., as Skolithos burrows have not been reported from the Unicoi Fm. This leads us to reevaluate the structural placement and interpretation of the Powhatan thrust, which appears to emplace younger over older rocks of the Chilhowee Group. In addition, we are using thin-section petrography to explore how the provenance of Chilhowee sandstones evolved from lower to upper units, and interpret environmental conditions that existed during the establishment of the Cambrian passive margin.